The invention relates to a pyrometer comprising a thermal radiation detector and an optical system for concentrating thermal radiation originating from an object surface area on the detector, said pyrometer being further provided with an emissivity meter comprising a radiation source supplying measuring radiation and a measuring radiation detector for convening the measuring radiation reflected by the surface into an electric signal. The invention also relates to a laser processing apparatus provided with such a pyrometer and to a method of controlling the temperature of a surface by means of the pyrometer.
A pyrometer measures the temperature of an object by measuring the quantity of thermal radiation from an object surface and convening it into a temperature of the surface. The relation between the quantity of thermal radiation and the temperature is dependent, inter alia, on the emissivity of the surface. The lower the emissivity of the surface, the less thermal radiation the surface emits at a given temperature. For a correct determination of the temperature it is thus necessary to know the emissivity value. In many processes in which temperatures are measured, reasonably invariable conditions prevail, such as, for example in ovens, and the emissivity has a constant value. Consequently, its value is to be determined only once. In processes in which considerably varying temperatures are used, for example in laser soldering, the emissivity varies during the process. This variation may be caused by a change of colour of the surface during the process or by a change of the roughness of the surface due to, for example melting of this surface, or by chemical changes, or only by the variation of the surface temperature. To perform a correct temperature measurement in such processes, the emissivity of the surface is to be measured simultaneously with the measurement of thermal radiation. For a correct determination of the temperature, the emissivity must be measured at the same wavelength position at which the thermal radiation is measured. For measuring the emissivity the the emissivity value is generally taken to be equal to 1 minus the value of the reflection coefficient, provided that the surface has a low transmission.
A pyrometer of the type described in the opening paragraph is known from the article "Der Pyrolaser" by R. Porter, published in Sensor Magazin no. 3, 1990, pp. 13 to 17. This known pyrometer measures the thermal radiation of a surface on which the pyrometer is focused and whose temperature must be measured. During the thermal radiation measurement, the pyrometer collimates measuring radiation from a pulsed laser on the surface. The measuring radiation reflected by the surface towards the pyrometer is detected and the pyrometer determines the value of the emissivity from the intensity of the reflected radiation. The magnitude of the thermal radiation of the surface measured between consecutive laser pulses is corrected for the emissivity, whereafter a corrected temperature of the surface is obtained.
A drawback of the known pyrometer is that it functions only properly if the surface is diffusely reflecting. If the surface reflects specularly and is not perpendicular to the laser beam, measuring radiation will not reflect in the pyrometer and the pyrometer will then erroneously determine an emissivity of 1. The temperature subsequently determined from the thermal radiation and this emissivity will be too low. It is found that a great many surfaces reflect only a part of incident radiation diffusely and another part specularly. When performing pyrometry on these surfaces, said drawback will thus occur to a greater or lesser extent.